The subject of the present invention is a pair of plates of a hard material (a socalled ceramic material or the like), intended for forming the flow control means of a faucet of the socalled screw type, and having an extended control field.
The screw faucets having a pair of hard material plates comprise a fixed plate traversed by a through passage opening, and a movable plate superimposed to and contacting the fixed plate, having a passage opening as well and connected to control means which allow to rotate the same. The passage openings of the fixed plate and of the movable plate are so designed that in a certain position of the movable plate (closure position) the passage openings of both plates do not register, and therefore a flow brought to pass through the pair of plates is completely intercepted, in another position of the movable plate (position of complete opening) the flow enjoys the maximum passage section allowed by the mutual cooperation of the passage openings of the two plates, which usually corresponds to the passage opening of the movable plate. In the positions intermediate the above mentioned ones the flow enjoys a gradually variable passage section resulting from the partial superimposition of the passage openings of both plates. This allows controlling the flow rate from zero up to the maximum flow rate allowed by the faucet.
In most faucets of the mentioned kind the control angle which leads from the closure position to the position of complete opening amounts to 1/4 of a turn or to 1/2 turn. The choice of such angles is determined by the fact that they may correspond to very simple shapes of the passage openings of the plates, whose manufacture is therefore easy. But such relatively reduced control angles are not favorable for a careful flow control by the user, and for this reason some faucets of this kind have been proposed, wherein the control angle allowed is of 3/4 of a turn. This has been obtained by giving the passage opening of the movable plate the shape of a sector of a circle extending on 1/4 of a turn, and the passage opening of the fixed plate a shape comprising a first portion being a sector of a circle extending on 1/4 of a turn, corresponding to the passage opening of the movable plate, a second connection portion also extending on 1/4 of a turn, and a third portion shaped as a curved channel of uniform width, located near the plate center and extending on nearly 1/4 of a turn. This shape of the passage openings of the plates, however, does not give rise to a satisfactory variation law of the flow rate depending upon the angular displacement of the movable plate. In effect, within the field of the very small flow rates a great control sensitivity is obtained, which however in most cases is not needed, whereas the control sensitivity is rather reduced within the field of the middle and middle-high flow rates, namely the field in which usually the faucet is used the most. Moreover, in order to ensure sealing in the closure position, the edges which limit the passage openings of the plates should superimpose the one another at least by a fixed width, and due to the fact that the third portion of the passage opening of the fixed plate is located near the plate center, to this fixed superimposition width corresponds a rather large angle, which cannot be used for the faucet control stroke.